Virtualmin is not your operating system and has nothing to say about how much of the disk is used.
When you install Ubuntu, you have the opportunity to specify partition sizes. The installer defaults to a rather stupid 100GB, which it splits up. The simplest thing is to tell it to not be stupid during installation.
But, you can also expand it later. / is on a volume, so you can expand the logical volume and then expand the filesystem. There’s tons of documentation out there about LVM and filesystems. This is a pretty good article from Red Hat:
Note that’s a Red Hat system with XFS filesystem. Ubuntu will probably use ext4 as its filesystem, and, so you’d resize it with resize2fs instead of grow_xfs.
Webmin has a Logical Volume Management module in the Hardware category, I’m not sure how easy it is to use as I’ve never used it (I’m more comfortable using the command line tools for volumes and filesystems). And, it also has a Disk and Network Filesystems module in System category, but I don’t think it knows how to resize. Resizing filesystems is a pretty unusual thing on servers. Usually you give it the whole disk at install time and never think about it again.